Pneumatic counterbalance units have come into widespread use in recent years, being used on various automotive components such as hoods, trunk lids, hatch backs and on other items such as machine covers, doors and hatches. Such pneumatic counterbalances are rather simple pneumatic piston/cylinder arrangements having a bleed by-pass through or past the piston similar to shock absorbers and oleo struts used on vehicles and air craft for many years. The well-known principle of operation is based on differential pressure resulting from the differential in cross section areas of the two chambers within the cylinder caused by the piston rod on one side of the piston which reduces the area on that side of the piston. Pneumatic counterbalance units, prior to this invention, have been pressurized subsequent to assembly, past the shaft seal, or through a fill valve or a fill port which is sealed after pressurization. Prior art units utilize orifice bleed passages through the piston or through the piston rod from one side of the piston to the other side and more sophisticated units have a valving device which varies the bleed rate depending on the direction of movement of the piston. Other forms of counterbalance devices provide a dual force by using a supplemental compression coil spring which becomes effective at an intermediate position of the piston rod retraction stroke.
Examples of prior art counterbalance units can be seen in the following U.S. Patents: U.S. Pat. No. 1,994,722 to O. W. Landerslager for Resilient Device; U.S. Pat. No. 2,774,446 to Bourcier de Carbon for Shock Absorbers: U.S. Pat. No. 2,788,867 to A. R. Causse for Shock Absorber; U.S. Pat. No. 3,207,498 to E. Wustenhagen et al. for Gas Spring; U.S. Pat. No. 3,222,047 to F. Tuczek for a Pneumatic Suspension Unit; U.S. Pat. No. 3,589,701 to D. W. Gee for Vehicle Suspension (Gas Spring); U.S. Pat. No. 3,856,287 to Freitag for Piston Rod Seal For Adjustable Pneumatic Spring; U.S. Pat. No. 3,868,097 to P. H. Taylor for Liquid Spring; U.S. Pat. No. 3,913,901 to W. Molders for Resilient Supportng Column; U.S. Pat. No. 3,963,227 to W. Molders for Gas Spring with Duel Damping; U.S. Pat. No. 4,030,715 to H. O. Duran for Pneumatic Shock Absorber; U.S. Pat. No. 4,064,977 to D. F. Taylor for Fluid Amplifier Shock Absorber having De Laval Nozzle; U.S. Pat. No. 4,098,302 to Freitag for Method for Charging Pneumatic suspension Element; and U.S. Pat. No. 4,108,423 to J. J. Skubal for Gas Spring. Most of the foregoing patents disclose various types of piston unit orifices and of different kinds of seals between the piston rod and cylinder, some being plurality O-rings or flat washers or a combination of both kinds and some, e.g., Taylor and Duran use chevron seals. Duran shows an anti-friction ring on the piston with plural peripheral notches providing free-flow by-pass of fluid. Some disclose methods of pressurizing, e.g., De Carbon shows a valved inlet; Tuczek shows a multiple inlet system with lines and accumulators; Freitag U.S. Pat. Nos. 3,856,287 shows a ball check valve; and Freitag U.S. Pat. No. 4,098,302 charges gas and liquid past the seals. The Taylor U.S. Pat. No. 3,868,097 teaches a structure and method of assembly for a liquid spring.